The grade-separated pedestrian systems built in the 20th century have a variety of names: skyways, skywalks, pedways, footbridges, the +15, and the Ville Souteraine. But they have one thing in common — they have radically altered the form and spatial logic of cities around the world…
However, recent proposals suggest that a synthesis of those three epochs of multilevel urbanism is indeed possible, if not inevitable. For example, SOM’s 2012 proposal for Grand Central Station envisions a high-density cluster of towers that would surround the station with densely layered transportation systems, including pedestrian walkways and concourses, mass transportation, shopping malls, rooftop gardens, and a circular observation deck that hovers over Grand Central like a halo.